The invention is aimed more specifically at mills used for grinding cement (clinker) by a dry route, coal, limestone and ores by the wet or dry route. These mills comprise a metal cylindrical shell ring rotating about its longitudinal axis and containing a grinding charge comprising grinding bodies, generally balls, but which may also comprise cylindrical pebbles, spherical pebbles, etc. of various sizes. The material for grinding is introduced on one side of the mill and, as it progresses toward the outlet on the opposite side, it is crushed and ground between the grinding bodies.
A conventional mill is generally divided, in the axial direction, by means of a diametral partition wall, into two successive chambers. The first chamber in which the coarse crushing of the material takes place contains grinding balls which generally have a diameter of between 60 mm and 90 mm. The second chamber, in which fine grinding takes place, contains grinding balls of a diameter generally between 15 mm and 60 mm. Besides these two-chamber mills there are also mills with just one chamber which contain grinding bodies of different diameters and in different quantities according to the diameter.
In the second chambers of conventional mills or in single-chamber mills, it is well known that it is necessary to have self-classifying liners, that is to say liners which, as the mill rotates about their axis, automatically classify the grinding bodies according to their size and more specifically classify the large grinding bodies at the inlet of the grinding chamber and the smaller ones toward the outlet of this same chamber, this being so that the weight and the size of the grinding bodies decrease as the particle size of the material progressing through the grinding chamber reduces and becomes finer. In this way, over the length of the grinding chamber, the size of the grinding bodies is suited to the particle size and fineness of the material that is to be ground. This generally makes it possible to reduce the power consumption per ton of ground material by 10 to 20%.
There are currently in existence various types of self-classifying liners. One of them has a saw-tooth shape in the axial direction of the mill, that is to say comprises a succession of cone frustums along the length of the mill which converge toward the outlet and have a slope directed toward the inlet of the grinding chamber. The plates which form these liners have a relatively high mean thickness and are therefore fairly heavy. This high thickness also leads to a loss of working volume in the grinding chamber and therefore, in some cases, to an inability to absorb all the available power of the motor. These liners are also very sensitive to the grains, that is to say that when there is a certain build-up of very hard grains (about 6 to 12 mm) in the regions where the small grinding bodies are located, classification is highly disrupted, it being possible for this disruption to go so far as to classify the bodies in reverse order, that is to say to send small bodies back toward the inlet and large bodies toward the outlet.
In another type of liner as described in document BE 09301481, the plates have corrugations which may be inclined by an angle of 15 to 30° with respect to the generatrix of the mill. The purpose of inclining these corrugations is to create a screw effect to act on the grinding charge and on the material to be ground. What happens is that when the mill turns, the large grinding bodies generally, for the most part, find themselves at the periphery of the grinding charge and the purpose of inclining the corrugations is to push these grinding bodies by a screw effect back toward the inlet of the grinding chamber. In practice, the classification desired in this way is, however, very difficult and often haphazard. The plates are also relatively heavy and the classification effect reduces as the corrugations gradually wear. These corrugations cannot be excessively pronounced because if they are there is discontinuous pickup, that is to say excessive pickup during which the outer layers of the grinding charge are picked up to the region of the top of the mill and drop back onto the liner instead of falling and rolling along the foot of the grinding charge. These liners are, in practice, very little used.